A subscriber line which is an information transmission line is widespread in enterprises, stores and houses as an infrastructure for Internet access by advancement of the information communication technology. Conventionally, digital transmission using a metallic subscriber line uses a basic rate interface of the ISDN (integrated services digital network) or an ADSL (asymmetric digital subscriber line). In recent years, an optical subscriber line is widespread and the metallic subscriber line is switched to the optical subscriber line. However, in a region where the optical subscriber line is not widespread, a region where the optical subscriber line is hardly widespread, and collective housing where the optical subscriber line is not introduced, the metallic subscriber line is used as an information transmission line still at present.
The ADSL is widespread as an Internet access means faster than the ISDN. The ADSL is a service using a frequency band having a large transmission loss, so that as a terminal is separated from a central telephone exchange, the deterioration in an electric signal is increased. Therefore, a problem arises that only the terminal located within a distance of 6 to 7 km from the central telephone exchange can enjoy the service. Further, in the ADSL, as indicated by the name, the speed on the upward is asymmetric to the speed on the downward. The main use of the ADSL in each house is to access the website of the Internet, so that the downward with large communication traffic is set at a high speed. Therefore, the ADSL is unsuitable for a means for improving the communication speed of the upward.
This problem of the ADSL can be solved by use of the optical subscriber line. However, a user who cannot use the optical subscriber line makes several contracts of the ISDN line to increase the communication speed of the up line, in the present state.
On the other hand, the ADSL adopts an ATM (asynchronous transfer mode) communication system. The ADSL using the ATM communication system premises the transmission and reception of data converted to an IP (Internet protocol) packet. Thus, it is necessary to convert PCM (pulse code modulation) data of voice etc. to the IP packet. Therefore, a terminal accommodation apparatus for connecting a telephone terminal to the ADSL needs a function (a VoIP processing unit) for converting a voice signal to the IP packet represented by a VoIP (Voice over IP) (refer to Japanese Patent Application disclosure 2006-203876).
However, the VoIP processing unit is comparatively expensive. Therefore, if each terminal accommodation apparatus installs an IP conversion unit, a problem arises that the cost for providing services is increased as a whole communication system.